The invention relates to a cooling system for an internal combustion engine with two-stage supercharging, including a charge-air line in which are provided a first compressor of a first turbocharger and a second compressor of a second turbocharger downstream of the former, a first charge-air cooler being provided between first and second compressor, and a second charge-air cooler downstream of the second compressor.
A cooling system of an internal combustion engine with two-stage supercharging of the above kind is disclosed in JP 62-085123 A. The first charge-air cooler or intercooler between first and second compressor will lower the charge-air temperature and thus prevent the second compressor from overheating. The cooling system does not provide for any charge control in dependence of the operating state of the engine, however. For this reason the turbochargers will not always reach optimum efficiency. A similar type of cooling system is described in DE 199 48 220 A1.
DE 39 33 518 A1 presents an internal combustion engine with a sequential turbocharger system comprising a first-stage high-volume turbocharger and a second-stage low-volume turbo-charger. By means of an intake bypass valve the second-stage low-volume turbocharger may be bypassed. This known kind of cooling system includes a charge-air cooler downstream of the first compressor, but no intercooler between first and second compressor. Due to the high exit temperature of the air leaving the first compressor, the second compressor, and most of all the compressor impeller, will be subject to high thermal loads, especially if a conventional impeller of cast aluminum is employed. Conventional impellers of cast aluminum are prone to low-cycle fatigue due to overheating. When the speed of the compressor impeller is increased tensile stresses in the hub region will result. When the speed is reduced the shift in stress will lead to compression stresses in the hub region. With a defined number of cycles and critical impeller dimensioning such fluctuating loads will destroy the impeller. Use of an intercooler can prevent this. Similar cooling systems are described in U.S. Pat. Nos. 5,020,327 A, 5,142,866 A, and 5,408,979 A.
U.S. Pat. No. 5,492,167 A discloses a cooling system with a liquid cooler and a charge-air cooler, which latter may be placed in front of the liquid cooler.
JP 2000-120439 A shows an engine cooling design with an intercooler being provided in front of a liquid cooler.
In U.S. Pat. No. 6,029,345 A a cooling system for an automotive vehicle is disclosed, which comprises a condensor, a charge-air cooler and a radiator. Condenser and charge-air cooler are disposed in front of the radiator in flow direction of the cooling air.
In conventional cooling systems charge-air cooler or coolant cooler are directly attached to the vehicle frame. This demands the use of flexible elements in the charge-air and/or cooling lines in order to compensate relative movements between engine and frame. Besides, flexible elements and/or correspondingly large spaces must be provided between fan cowling and engine. Such cooling systems are disclosed in U.S. Pat. Nos. 4,213,426 A and 4,774,911 A.
An engine cooling system with a coolant radiator is known from U.S. Pat. No. 5,597,047 A, which is held by a radiator support frame attached directly to the engine block. Dampening modules are disposed between the radiator and the support frame attached to the engine. The advantage of such cooling assemblies which are rigidly connected to the engine is that connecting lines may be short.
It is the object of this invention to develop an efficient cooling system for an internal combustion engine featuring a space-saving, light-weight design with a reduced number of parts and long life.
According to the invention this object is achieved by disposing at least one of the two charge-air coolers upstream of the coolant cooler in flow direction of the cooling air, and by providing the second charge-air cooler above or beside the first charge-air cooler. Disposing the charge-air coolers in front of the coolant cooler will safe space and help obtain efficient cooling of the charge air.
Due to the two-stage supercharging design comparatively low-cost materials may be employed. The first charge-air cooler will allow the charge-air temperature between the two compressors to be reduced to a level which will permit the use of a conventional impeller of cast aluminum even for the second compressor without adverse effects on its impeller life due to excessively high temperatures of the intake air upon entry into the second compressor. As a consequence the problem of low-cycle fatigue of the compressor impeller will be avoided.
By disposing the second charge-air cooler above the first charge-air cooler it will be possible to save space and obtain optimum cooling of the charge air.
According to an especially preferred variant the inlet and outlet of at least one charge-air cooler, and preferably the first charge-air cooler, are provided on one and the same side relative to a vertical plane extending through the center of gravity of the charge-air cooler in the flow direction of the cooling air. Advantageously, at least one charge-air cooler, and preferably the first charge-air cooler, is provided with a separating wall through the center of gravity, which acts as a partition between incoming and outgoing charge-air stream, so that an essentially U-shaped flow path is obtained for the charge air flowing through the charge-air cooler. In this way the charge-air line between the first charge-air cooler and the second compressor may be kept very short, thus saving material and reducing flow losses.
In a preferred embodiment of the invention it is proposed that the cooling assembly comprising first and second charge-air cooler and the coolant cooler be rigidly attached to the engine. This arrangement will allow for small distances between the individual parts, as relative movements need not be taken into account. In this way a very compact cooling system will be obtained. Moreover, flexible elements in the charge-air lines will become superfluous, and costs and maintenance may be reduced. No flexible elements, such as rubber hoses of low fatigue strength will be required. Nor need the cooling assembly be attached to a supporting frame with the use of rubber bearings. For installation in the vehicle the rigidly connected cooling assembly is lifted together with the engine and the gearbox into a flexible drive unit supporting frame. Since no flexible elements are used in the charge-air lines the individual coolers and lines will not be subject to gas reaction forces. Separate support elements and the like will thus become superfluous.
Alternatively it could be provided that the cooling assembly be rigidly attached to the vehicle frame. In this instance flexible hose elements must be provided in the charge-air line between first compressor, first charge-air cooler, second compressor, second charge-air cooler, and an intake manifold.
In further development of the invention the proposal is put forward that the second turbocharger be bypassed in a controlled manner, preferably at the exhaust end. Bypassing the second compressor, i.e., the high-pressure compressor, by means of the bypass line and at least one or several valves in the instance of large exhaust volumes, such as at high loads or engine speeds, will permit both turbochargers to run at optimum operating conditions, thus significantly raising engine efficiency compared to uncontrolled serial charging systems. The compact high-pressure turbine/compressor combination will pick up speed fast even in the instance of low exhaust energies. In this way the internal combustion engine will show good response.
For generating the cooling-air stream a preferably axial-flow type fan is provided, which is driven by a switchable coupling to regulate the amount of cooling air. If only a small amount of cooling air is required the driving power may be reduced accordingly. The coupling is preferably provided with a visco-thermoelement. Alternatively, the coupling could be controlled externally via at least one cooling-air temperature sensor and an electronic control unit.